The large signal model of a junction diode is represented by Equations 3.3 and 3.4. The non-linear current source shown in Figure 3.2 is represented in VHDL-AMS in a similar fashion as shown for the static model of a junction diode, using simultaneous if statement and break statement. The charge-storage effects in a diode are modeled using a capacitance(
). This capacitance is described as a sum of two capacitances(
), in VHDL-AMS, as shown below:
where cd is the diffusion capacitance, cj is the junction capacitance, capd is the total capacitance of the diode, icd is the current through the capacitor and vd is the voltage across the capacitor(or diode junction).
Figure 4.4: Half-wave rectifier circuit
To validate the large-signal characteristics of a diode we can set up many circuit arrangements [23] such as voltage doubler, DC restorer, diode limiter, half-wave and full-wave rectifier circuits. As a part of this research the large-signal model of the diode is validated using all the above mentioned circuit arrangements. To be concise, this thesis presents only the half-wave rectifier circuit arrangement as shown in Figure 4.4 to validate the large-signal characteristics of a diode. To obtain the transient characteristics of a half-wave rectifier we apply a AC power supply voltage of 120 V-RMS(169 V peak-peak) at 60 Hz. The purpose of the transformer in Figure 4.4 is to step down the main AC power supply voltage to a 12V-peak level. The transformer is modeled as a mutual inductor with coupling factor very close to unity. The following VHDL-AMS code describes the mutual inductor used in the above circuit.
where m is the mutual inductance of the transformer, calculated from the coupling factor(k) and primary and secondary inductances(lp, ls). ilp'dot and ils'dot represent the first order differentials of primary and secondary inductances with respect to time, respectively.
Table 4.2: Diode large-signal parameter values used in determining the transient characteristics of a half-wave rectifier circuit
Figure 4.5: Half-wave rectifier circuit transient characteristics
The diode used in the circuit is a commercial type 1N4148 diode and the purpose of the isolation resistance(100 M
) is to provide a DC path between the secondary side of the transformer and ground. The specifications of 1N4148 diode are listed in Table 4.2. The half-wave rectifier circuit is described both in VHDL-AMS and SPICE and is validated using respective simulators. The resulting plots are shown in Figure 4.5. The first plot in the figure shows the voltage across the primary side of the transformer(vlp) and the second plot shows the voltage across the secondary side of the transformer(vls) and the rectified voltage across the load resistor(vout). The results obtained from SPICE and SEAMS appear to be in close agreement, which implies the equal performance of the diode model in SPICE and VHDL-AMS.